It's simple, and if you've played the beta (I don't play the game often enough to actually have a rank, which was one of the requirements) you know where we're going.

Quick runthrough:

1. Open game properties, either by right clicking the game list or the gear icon in the game view in the library and clicking Properties...
   
2. Go into the Betas menu
   
3. Choose csgo_demo_viewer - 1.38.7.9 (Not the 2013 version, I couldn't even get that one to launch)
   
4. Let it download and install
   
5. You can now choose to launch CS2 or CSGO "Demo_viewer" when clicking Play

It works with bots and community servers, but matchmaking is gone.
Version with images:

• Right click CS2 in game list or click the gear icon and choose Properties...
  

• Betas menu, choose the one displayed here
  

• Let it download and install
  

• It should now give you the option. If not, gear icon.
  

• It's gonna complain, but it launches just fine
  

• Of course, matchmaking won't work.
  

• Oh but what's this?
  

• CS 1.6? No! It's a community server browser! Just like every single damned multiplayer Source and GoldSrc game!
  

All other forms of playing CSGO are still available, as for now. If Valve ever nerfs this version to only be able to play demos, I will be very reasonably angered and I might even start buying games on GOG... O_o
But it still works, try launching a game with bots.